Ph buffer how does it work

This particular resource used the following sources:. Skip to main content. Aqueous Reactions. Search for:. Learning Objective Explain the composition of buffer solutions and how they maintain a steady pH. Key Points A basic solution will have a pH above 7. Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet. October 22, October 16, Because of this behavior, strong acids and their conjugate bases do not usually form buffer solutions.

So too do strong bases. As a result, buffer solutions usually consist of a mixture of weak acids and their conjugate bases and weak bases and their conjugate acids. Here is an example of a weak acid—acetic acid with its conjugate base—acetate ion. Together, they can form a buffer:. To understand how buffer solution resist changes in pH, we must first consider the dissociation of weak acid in water.

However, since HA is a weak acid, only some of its molecules will dissociate. As a result, we can write the following equations to describe the dissociation of weak acid in water:. From equation 1, you will notice that the weak acid, HA donates a proton to water and then turns into its conjugate base, A —.

Thus, the acid, HA is in equilibrium with its conjugate base, A —. Similarly, from equation 2, you will notice that the weak acid, acetic acid is in equilibrium with its conjugate base, acetate ion.

When you add small amounts of strong base OH — to a buffer, the buffer will resist changes in its pH by sending an equal amount of its weak acid to donate a proton to the base.

Once the base accepts the proton, it turns into water, while the weak acid turns into its conjugate base. If you need to know about calculations involving buffer solutions, you may be interested in my chemistry calculations book. An alkaline buffer solution has a pH greater than 7. Alkaline buffer solutions are commonly made from a weak base and one of its salts. A frequently used example is a mixture of ammonia solution and ammonium chloride solution.

If these were mixed in equal molar proportions, the solution would have a pH of 9. Again, it doesn't matter what concentrations you choose as long as they are the same. A buffer solution has to contain things which will remove any hydrogen ions or hydroxide ions that you might add to it - otherwise the pH will change. Acidic and alkaline buffer solutions achieve this in different ways. Adding sodium ethanoate to this adds lots of extra ethanoate ions. According to Le Chatelier's Principle, that will tip the position of the equilibrium even further to the left.

Note: If you don't understand Le Chatelier's Principle , follow this link before you go any further, and make sure that you understand about the effect of changes of concentration on the position of equilibrium.

The buffer solution must remove most of the new hydrogen ions otherwise the pH would drop markedly. Hydrogen ions combine with the ethanoate ions to make ethanoic acid. Although the reaction is reversible, since the ethanoic acid is a weak acid, most of the new hydrogen ions are removed in this way.

Since most of the new hydrogen ions are removed, the pH won't change very much - but because of the equilibria involved, it will fall a little bit. This time the situation is a bit more complicated because there are two processes which can remove hydroxide ions. The most likely acidic substance which a hydroxide ion is going to collide with is an ethanoic acid molecule. They will react to form ethanoate ions and water.

Note: You might be surprised to find this written as a slightly reversible reaction. Because ethanoic acid is a weak acid, its conjugate base the ethanoate ion is fairly good at picking up hydrogen ions again to re-form the acid. It can get these from the water molecules. You may well find this reaction written as one-way, but to be fussy about it, it is actually reversible!

Hydroxide ions can combine with these to make water. As soon as this happens, the equilibrium tips to replace them. This keeps on happening until most of the hydroxide ions are removed. Again, because you have equilibria involved, not all of the hydroxide ions are removed - just most of them. The water formed re-ionises to a very small extent to give a few hydrogen ions and hydroxide ions.

Adding ammonium chloride to this adds lots of extra ammonium ions. The most likely basic substance which a hydrogen ion is going to collide with is an ammonia molecule. They will react to form ammonium ions. Most, but not all, of the hydrogen ions will be removed. The ammonium ion is weakly acidic, and so some of the hydrogen ions will be released again.

Remember that there are some hydroxide ions present from the reaction between the ammonia and the water. Hydrogen ions can combine with these hydroxide ions to make water. As soon as this happens, the equilibrium tips to replace the hydroxide ions. This keeps on happening until most of the hydrogen ions are removed. Again, because you have equilibria involved, not all of the hydrogen ions are removed - just most of them.

Because the ammonia formed is a weak base, it can react with the water - and so the reaction is slightly reversible. That means that, again, most but not all of the the hydroxide ions are removed from the solution.

This is only a brief introduction. There are more examples, including several variations, over 10 pages in my chemistry calculations book.

This is easier to see with a specific example. Remember that an acid buffer can be made from a weak acid and one of its salts. Let's suppose that you had a buffer solution containing 0.